U.S. Pat. No. 5,796,214, U.S. Ser. No. 08/709,062, filed on Sep. 6, 1996 and U.S. Ser. No. 08/897,435, filed Jul. 21, 1997 all by the present inventor, discloses and claim ballasts for an electrodeless lamp. The ballasts include a d.c.-to-a.c. converter formed of a pair of serially connected switches having opposite conduction modes. For instance, one switch may be an n-channel enhancement mode MOSFET, and the other a p-channel enhancement mode MOSFET, with their sources interconnected at a common node. This allows a single control voltage applied to the gates, or control nodes, of the MOSFETS to alternately switch on one MOSFET and then the other. The foregoing ballasts allow the lamp to be in either an "on" state or an "off" state, but does not provide a matter of dimming the electrodeless lamp.
In existing electrode lamps, conventional methods continuously change the frequency of oscillation to control the amount of current flowing through an arc, and therefore the lumens output from the lamp. Attempting to apply this concept to electrodeless fluorescent lamp systems would result in overheating of the r.f. coil and the ballast switches. Additionally, conventional dimming methods will not produce a sufficient h-field to sustain a toroidal discharge when the arc current is reduced to less than 50% of rating. As the arc current decreases, the h-field decreases and the azimuthal e-field increases, causing the toroidal arc to extinguish and a longitudinal glow discharge to continue.
It would be desirable to provide a ballast, for an electrodeless lamp, which incorporates a dimming circuit to allow a range of dimming control for an electrodeless lamp.